We use Plone CMS (http://www.plone.org) and it's fantastic. Loads of options beyond standard wiki, easy to set up.
Edit: Quote from their site - "An eWeek Labs Analyst's Choice award winner, this open-source product is one of the best solutions period - for company portals and intranets"
- eWeek, April 2006 issue
Edit: Quote from their site - "An eWeek Labs Analyst's Choice award winner, this open-source product is one of the best solutions period - for company portals and intranets" - eWeek, April 2006 issue